This invention relates to reproducing apparatus, and more particularly, to apparatus for generating patterns from information stored in a computer or similar storage apparatus.
The fabrication of semiconductor integrated circuits requires repeated projection of light through different masks onto a semiconductor wafer coated with a photosensitive film. After each exposure and appropriate development, the film itself then constitutes a mask for permitting selective processing of the wafer, such as etching or diffusion. The photolithographic mask pattern may be prepared by a draftsman and then photographically reduced to a size appropriate for the production of minuscule integrated circuits.
The patent of Herriott et al. U.S. Pat. No. 3,573,849 granted Apr. 6, 1971 assigned to Bell Telephone Laboratories, Incorporated, describes a pattern generating system using laser beam exposure of a photographic film. The pattern is described entirely by digital information stored in a computer; that is, by trains of stored electrical pulses or "bits" each representing successive spots on a mask pattern that are either transparent or opaque. A positive pulse or one bit may represent a transparent spot, while the absence of a pulse, or zero bit, may represent an opaque spot. The stored information is used to modulate the laser beam which scans the photographic film.
The scanning operation is performed by a rotating polygonal mirror which reflects the beam to scan in an x direction, with the photographic film being periodically stepped in an orthogonal y direction. Because the beam is reflected from a single point on the moving mirror, the mirror rotation would normally give a non-uniform scanning velocity across the photosensitive medium; however, a unique scanning lens is included to adjust and linearize the light beam scanning velocity. Although laser beams are theoretically capable of extremely high resolution, such resolution in the Herriott et al. apparatus, and similar prior art machines, is limited by such factors as the resolution capabilities of the photosensitive medium, the scanning lens, and by the essential change of reflected direction of the writing beam during scanning. Accordingly, masks made by the Herriott apparatus are typically made initially to be 35 times larger than the size of the intended integrated circuit pattern, photographically reduced to a 10:1 ratio, and subsequently reduced again by a step-and-repeat camera to produce a multiple-array mask, each component containing a circuit pattern of the proper size.